The present invention relates generally to methods and apparatuses for adding solid alloying ingredients to molten metal and more particularly to the addition of solid, particulate alloying ingredients to a stream of molten metal descending from an upper container to a lower container.
It is oftentimes desirable to add alloying ingredients in solid, particulate form, such as shot, to a molten metal stream descending from an upper container, such as a ladle, to a lower container, such as the tundish in a continuous casting apparatus. It is desirable to add the alloying ingredients to the descending stream of molten metal because this facilitates the mixing of the alloying ingredients into the molten metal, such as molten steel.
It is desirable also to add the alloying ingredient as shot particles, because in that form the alloying ingredient can be precisely metered, and there is rapid dissolution and dispersion of the alloying ingredient in the molten metal.
Certain alloying ingredients for molten steel, such as lead, bismuth, tellurium and selenium, typically added to molten steel to improve the machineability of the resulting solid steel product, have relatively low melting points compared to steel and are prone to excessive fuming or oxidation when added to molten steel, particularly when these alloying ingredients are in the form of shot. One expedient, for coping with the fuming and oxidation problems which arise when adding these ingredients to molten steel, comprises enclosing the descending stream of molten steel within a vertically disposed, tubular shroud having a lower end which extends below the top surface of a bath of molten steel in the tundish. The alloying ingredient is directed into the descending stream inside the shroud. The shroud protects the descending stream and the alloying ingredient against exposure to the outside atmosphere surrounding the ladle and the tundish.
When the solid alloying ingredient is introduced into the descending stream of molten steel in the form of shot, the shot can be mixed with a compressed, non-oxidizing gas, such as argon or nitrogen, which acts as a transporting or carrying medium for the shot. The mixture of shot and compressed gas is directed toward the descending stream of molten steel through a nozzle having an outlet end exposed to the interior of the shroud. When a compressed gas is employed in this manner, the compressed gas expands within the shroud and has a cooling effect therein. Furthermore, the metallic alloying ingredient undergoes a change in state as it enters the descending stream of molten steel, changing from solid shot to liquid (and some of that possibly to vapor), and this change of state absorbs heat and has an additional cooling effect within the shroud.
The addition of alloying ingredient in the form of shot, to a descending stream of molten steel, inside a surrounding shroud, employing compressed inert gas as a carrying medium, is disclosed in Rellis, et al., U.S. Pat. No. 4,602,949 ('949) entitled "Method and Apparatus for Adding Solid Alloying Ingredients to Molten Metal Stream", and the disclosure thereof is incorporated herein by reference.
A problem which can arise when employing an arrangement of the type described in the Rellis, et al. '949 patent is the build-up of a skull of steel inside the shroud. This is caused by the cooling effect of the expanding gas on droplets of molten steel which originate in the descending stream and impinge against the interior of the shroud. The cooling effect of the expanding gas causes the droplets to solidify on the interior of the shroud resulting in the build-up of the aforementioned skull. This is undesirable because skull build-up eventually can cause a blockage of the nozzle outlet end, thereby preventing the shot from entering the descending stream of molten steel.
One expedient for coping with the problem of skull build-up within the shroud is described in Peters, et al., allowed U.S. application Ser. No. 169,884 filed Mar. 18, 1988 and entitled "Method and Apparatus for Adding Liquid Alloying Ingredient to Molten Steel". In this expedient, the form of the alloying ingredient is changed from solid particulate to molten. As a result, no pressurized carrier gas is needed to convey the alloying ingredient to the interior of the shroud, and the cooling effect resulting from the expansion of the compressed carrier gas is eliminated. This expedient, however, requires auxiliary equipment to melt the alloying ingredient, to hold the alloying ingredient in molten form, and to pump or otherwise deliver the molten alloying ingredient to the shroud interior. The disclosure of said allowed Peters, et al. application is incorporated herein by reference.
When the alloying ingredient is added in the form of shot mixed with a carrier gas, the nozzle which directs the shot particles has a downstream outlet end which is exposed to the shroud interior. The shroud is composed of refractory material, and the temperature within the shroud interior is relatively high despite the cooling effect of the expanding carrier gas. The high temperature causes the nozzle to heat up, and there is a decreasing temperature gradient extending upstream in the nozzle from the nozzle outlet end. This can cause premature melting, within the nozzle, of the shot which has a relatively low melting point. The temperature gradient in the nozzle can also cause the shot, at locations upstream of the nozzle outlet end, to become sticky or tacky. As a consequence, there can be a build-up of alloying ingredient within the nozzle, at a location upstream of the nozzle outlet, eventually causing a blockage within the nozzle.
To cope with the problem described in the preceding paragraph, a special nozzle construction was developed, and this is described in Rellis, et al., U.S. Pat. No. 4,747,584 ('584) entitled "Apparatus for Injecting Alloying Ingredient Into Molten Metal Stream", and the disclosure thereof is incorporated herein by reference. The nozzle described in Rellis, et al. '584 is composed of inner and outer tubular members. The mixture of transport gas and metal shot is conducted through the inner tubular member. A cooling fluid is circulated through the outer tubular member to cool the inner tubular member. Baffles and a passageway are provided between the two tubular members to define a path along which the cooling fluid flows from an inlet location adjacent the upstream end of the nozzle downwardly towards the downstream end of the nozzle and then back upwardly toward the upstream end of the nozzle where the cooling fluid is withdrawn from the nozzle.
In the arrangements employed in the two Rellis, et al. patents, the descending stream of molten steel was introduced into the shroud through a vertically disposed conduit having a lower outlet end located near the upper end of the shroud. The lower end of the shroud was desirably disposed below the top surface of the bath of molten steel in the tundish. Problems arose which restricted the extent to which the shroud's lower end could be submerged within the bath of molten steel, and as a result, the lower outlet end of the vertically disposed conduit was located relatively far above the top surface of the bath. This was undesirable because it increased the length of the unenclosed part of the descending stream, i.e., the part between the lower outlet end of the vertically disposed conduit and the top surface of the bath. The alloying ingredient is directed into the unenclosed part of the descending stream. It is desirable to maintain the unenclosed part of the descending stream as short as possible because the longer it is, the greater the danger of oxidation.
Attempts have been made to avoid the problem of skull build-up within the shroud by eliminating the shroud. Elimination of the shroud also enables the lower outlet end of the vertically disposed conduit, to be located closer to the top surface of the bath, thereby reducing the length of the unenclosed part of the descending stream. In these attempts, the alloying ingredient was added to the descending stream of molten metal with a nozzle directed toward the stream at an angle having a downward component. This nozzle has an upstream inlet end communicating with the downstream portion of a transporting conduit. The conduit's downstream portion extends upstream at the same angle as the nozzle and communicates with an upstream portion extending horizontally directly from the downstream portion at an angle thereto. When the nozzle was uncooled or insufficiently cooled, problems arose. These problems included overheating of the nozzle and of the transporting conduit and restrictions in the flow of material through the transporting conduit. Overheating of the nozzle or of the transporting conduit also caused the shot to burn up in the conduit or nozzle or to melt therein and cause blockages.